1. Field of the Invention
This invention relates to a system for measuring the diameter of optical waveguide fibers. More particularly, this invention relates to a circuit for increasing the precision with which such diameter measurements can be made.
The outside diameter of optical waveguide fibers must be precisely controlled since diameter variations can affect connector losses and the attenuation of optical waveguides. Diameter measurements must be performed on-line during the drawing process so that information derived therefrom can be employed to control the draw apparatus. In order to preserve the strength of optical waveguide fibers it is necessary to employ noncontact measurement. Once process disturbances have been eliminated or minimized, the controllability of the fiber drawing process appears to be limited only by the diameter measurement resolution; therefore, this resolution should be maintained as high as possible.
2. Discussion of the Prior Art
U.S. Pat. No. 3,982,816--L. S. Watkins discloses a technique for determining the diameters of successive axial portions of an optical fiber. Such technique, which is typically performed upon an axially advancing fiber during the fiber drawing process, involves the illumination of a newly drawn, axially advancing fiber with a radially directed beam of coherent, monochromatic radiation, thereby creating a forward scattering pattern of interference fringes. The pattern is examined over a predetermined range of scattering angles in order to determine the number of fringes present in the predetermined range. A succession of counts of such fringes provides information as to the diameters of successive axial portions of the fiber advancing past the beam of radiation.
Apparatus for detecting the fringe pattern is disclosed in U.S. Pat. No. 4,046,536--D. H. Smithgall and in the publication, D. H. Smithgall et al. "High-Speed Noncontact Fiber-Diameter Measurement Using Forward Light Scattering" Applied Optics, Vol. 16, No. 9, September, 1977, pp. 2395-2402. Both the Smithgall patent and publication teach a scanned diode array, suitably positioned with respect to the interference fringes, as the apparatus for sensing the presence of the fringes and generating an electrical signal corresponding thereto. The Smithgall publication discloses a circuit for generating pulses corresponding to the number of peaks and valleys in the fringe pattern as well as a counter for determining the fringe count and a microprocesser for converting the count to a fiber diameter display. The Smithgall patent teaches that the output of the counting electronics can be employed to control the speed of the drawing apparatus, thereby reducing the variation between the set point of the control system and the measured diameter indication.